Attention has been focused on a variable rate embedded speech encoding scheme having scalability as a speech encoding scheme that can flexibly support channel states which change with time (that is, transmission rate, error rate, and the like, at which communication is possible). Scalable encoding information can reduce coding information freely at an arbitrary node on the channel, and so scalable encoding information is effective in congestion control in communication which utilizes packet network typified by IP network. Against this background, various schemes appropriate for VoIP (Voice over IP) have been developed.
As such a scalable speech encoding technique, a scheme of using a encoding apparatus for telephone band speech signals in a core layer is known (for example, Patent Document 1). As a method of encoding telephone band speech signals, a scheme based on code-excited linear prediction (CELP) is widely used.
Non-Patent Document 1 discloses the technique of CELP.
    Patent Document 1: Japanese Patent Application Laid-Open No. HEI10-97295    Non-Patent Document 1: M. R. Schroeder and B. S. Atal, “Code-Excited Linear Prediction (CELP): High-Quality Speech at Very Low Rate,” Proc. IEEE ICASSP85, 25.1.1, pp. 937-940, 1985